利用MODIS数据估计中国黄东海区域测深参数

目的 现有关于漫衰减系数的研究大多是在490 nm波段建立反演模型,且未将相关研究与机载激光雷达测深能力建立联系,本文尝试获取测深参数532 nm漫衰减系数K_d(532)和透明度SD(Secchi disk depth),为机载双色激光雷达测深能力的评估和飞行方案的制定提供了重要技术参数。方法 首先分析了测深参数532 nm漫衰减系数K_d(532)和透明度SD对于评估机载双色激光系统测深能力的重要性。利用2003年春季中国黄东海区域的实测光学数据,对现有的漫衰减系数反演模式进行改进,建立了K_d(532)和K_d(490)=的线性关系以及SD和K_d(532)的幂函数关系。结果 根据2003年春季MODIS的490 nm漫衰减系数产品和上述函数关系获取了K_d(532)和SD参数。结论 本文获取测深参数K_d(532)和SD的方法能够用来有效评估该区域机载激光雷达的测深能力,准确性和精度依赖于实测光学数据的精度、分布和数量以及MODIS的K_d(490)产品的准确性。     

数据仓库系统中层次式Cube存储结构

区域查询是数据仓库上支持联机分析处理(on-line analytical processing,简称OLAP)的重要操作.近几年,人们提出了一些支持区域查询和数据更新的Cube存储结构.然而这些存储结构的空间复杂性和时间复杂性都很高,难以在实际中使用.为此,提出了一种层次式Cube存储结构HDC(hierarchical data cube)及其上的相关算法.HDC上区域查询的代价和数据更新代价均为O(log^dn),综合性能为O((logn)^2d)(使用C_qC_u模型)或O(K(logn)^d)(使用C_qn_q+C_un_u模型).理论分析与实验表明,HDC的区域查询代价、数据更新代价、空间代价以及综合性能都优于目前所有的Cube存储结构.     

k-Median近似计算复杂度与局部搜索近似算法分析

k-Median问题的近似算法研究一直是计算机科学工作者关注的焦点,现有研究结果大多是关于欧式空间和Metric空间的,一般距离空间k-Median的结果多年来一直未见.考虑一般距离空间k-Median问题,设d_max/d_min表示k-Median实例中与客户点邻接的最长边长比最短边长的最大者.首先证明d_max/d_min≤ω+ε的k-Median问题不存在近似度小于1+ω-1/e的多项式时间近似算法,除非,由此推出Metric k-Median问题不可近似到1+2/e,除非NP(∈)DTME(N^{O(log logn)}).然后给出k-Median问题的一个局部搜索算法,分析表明,若有d_max/d_min≤ω,则算法的近似度为1+ω-1/2.该结果亦适用于Metric k-Median,ω≤5时,局部搜索算法求解Metric k-Median的近似度为3,好于现有结果3+2/P.通过计算机实验,进一步研究了k-Median局部搜索求解算法的实际计算效果和该算法的改进方法.     

公交网络下的一种费用限制最小时态路径查询索引

私人交通网络下的最短路径查询主要考虑路径长度、行驶时间等因素,而公共交通网络下的路径查询需要考虑路径上相邻的边的时间顺序约束以及路径的费用.研究了公共交通网络下3种查询：给定起点、终点、时间区间和费用上限,查找在时间区间内不超过费用上限的最早到达路径、最晚出发路径和最短耗时路径.首先给出一种Dijkstra变种算法Dijk-CCMTP,在此基础上给出3类查询的查询算法.然后提出一种高效的索引结构ACCTL（approximate cost constrained time labelling）.ACCTL采用Dijk-CCMTP对图中的每个顶点预先计算部分从该顶点出发的和到达该顶点的基本路径.对于任意从起点s到终点d的查询,可以采用类似数据库表连接的方式从ACCTL中连接从a出发的和到达d的路径生成近似解,避免遍历原图搜索路径.ACCTL建立索引的时间复杂度是O（|V|·△_max·|E|·（log|E|+△_max））,其中,|V|表示顶点数,|E|表示边数,△_max表示顶点的最大度数.实验验证ACCTL索引支持的查询速度比Dijkstra的变种算法的查询速度快2~3个数量级,并分析了影响建立索引时间和空间大小的因素.     

Multi-log2N交换网络的性能分析模型及控制算法

高速多平面交换网络解决了其内部冲突问题,但需要相应的路由控制算法的辅助,否则,内部冲突不能彻底解决.这是因为包在输入级路由平面的选择不够恰当,容易导致路由冲突的产生.因此,根据冲突链路集的思想,给出一种Multi-log₂N交换网络的控制算法.该算法控制分组在路由平面间的选择,不仅能够适用于RNB和SNB,还能实现单播和多播的控制,保障Multi-log₂N完全实现无阻塞.另一方面,Multi-log₂N消除了内部的链路冲突,提高了交换速率,但对其交换性能缺乏系统的理论分析.给出一种基于嵌入式马尔可夫链的分析模型,对Multi-log₂N网络中队列的使用及分组在队列中的平均等待时间、平均队长等相关性能指标进行了系统的分析,为基于Multi-log₂N的光交换节点的设计提供了良好的理论依据.     

基于不相似性度量优化的密度峰值聚类算法

密度峰值聚类（clustering by fast search and find of density peaks,简称DPC）是一种基于局部密度和相对距离属性快速寻找聚类中心的有效算法.DPC通过决策图寻找密度峰值作为聚类中心,不需要提前指定类簇数,并可以得到任意形状的簇聚类.但局部密度和相对距离的计算都只是简单依赖基于距离度量的相似度矩阵,所以在复杂数据上DPC聚类结果不尽如人意,特别是当数据分布不均匀、数据维度较高时.另外,DPC算法中局部密度的计算没有统一的度量,根据不同的数据集需要选择不同的度量方式.第三,截断距离d_c的度量只考虑数据的全局分布,忽略了数据的局部信息,所以d_c的改变会影响聚类的结果,尤其是在小样本数据集上.针对这些弊端,提出一种基于不相似性度量优化的密度峰值聚类算法（optimized density peaks clustering algorithm based on dissimilarity measure,简称DDPC）,引入基于块的不相似性度量方法计算相似度矩阵,并基于新的相似度矩阵计算样本的K近邻信息,然后基于样本的K近邻信息重新定义局部密度的度量方法.经典数据集的实验结果表明,基于不相似性度量优化的密度峰值聚类算法优于DPC的优化算法FKNN-DPC和DPC-KNN,可以在密度不均匀以及维度较高的数据集上得到满意的结果;同时统一了局部密度的度量方式,避免了传统DPC算法中截断距离d_c对聚类结果的影响.     

离散多智能体系统中分布式事件触发的H∞滤波

研究了具有切换拓扑的离散时间多智能体系统的事件触发H_∞滤波问题.构造了一种新的分布式事件触发控制方案,以确定是否每个智能体应该传输当前采样数据到滤波器,从而有效的节约网络资源.考虑到网络诱导时延的存在,同时利用马尔可夫过程对网络拓扑的切换进行建模,采用事件触发控制方案并结合所提的H_∞滤波器,将分布式滤波误差系统建模成一个具有多时变时延的闭环系统.利用包含多区间上下界信息的Lyapunov-Krasovskii泛函和线性矩阵不等式方法,得到了保证闭环系统实现具有H_∞性能指标渐近稳定性的一些充分条件和H_∞滤波器参数的设计方法.最后,通过两个数值算例说明了该方法的有效性.     

独立多处理机任务静态调度问题的近似算法

研究独立多处理机任务静态调度问题P_m|fix|C_max,即在m个处理机系统中调度n个多处理机任务,每个任务指派到所需一组处理机上不可剥夺地执行.该问题应用广泛但早已证明为NP难问题,而且也不存在常数近似算法.分析了问题P_m|fix|C_max和其中所有任务都是单位处理机时间的特殊情形P_m|fix,p=1|C_max的调度,并利用实例划分(split scheduling,简称SS)、首次满足优先(first fit,简称FF)和最大宽度优先(large wide first,简称LWF)等方法,构造了问题P_m|fix,p=1|C_max的√2m +1近似算法和问题P_m|fix|C_max的2√m 近似算法,优于目前已有文献的最好结果.     

取定s的严格d-正则随机(3,2s)-SAT问题的可满足临界

3-CNF公式的随机难解实例生成对于揭示3-SAT问题的难解实质和设计满足性测试的有效算法有着重要意义.对于整数k>2和s>0,如果在一个k-CNF公式中每个变量正负出现次数均为s,则称该公式是严格正则（k,2s）-CNF公式.受严格正则（k,2s）-CNF公式的结构特征启发,提出每个变量正负出现次数之差的绝对值均为d的严格d-正则（k,2s）-CNF公式,并使用新提出的SDRRK2S模型生成严格d-正则随机（k,2s）-CNF公式.取定整数5<s<11,模拟实验显示,严格d-正则随机（3,2s）-SAT问题存在SAT-UNSAT相变现象和HARD-EASY相变现象.因此,立足于3-CNF公式的随机难解实例生成,研究了严格d-正则随机（3,2s）-SAT问题在s取定时的可满足临界.通过构造一个特殊随机实验和使用一阶矩方法,得到了严格d-正则随机（3,2s）-SAT问题在s取定时可满足临界值的一个下界.模拟实验结果验证了理论证明所得下界的正确性.     

具有模态词□φ=□1V□2φ且可靠与完备的公理系统

提出具有模态词□φ=□₁V□₂φ的命题模态逻辑,给出其语言、语法与语义,其公理化系统是可靠与完备的,其中,□₁与□₂是给定的模态词.该逻辑的公理化系统具有与公理系统S5相似的语言,但具有不同的语法与语义.对于任意的公式φ,□φ=□₁V□₂φ;框架定义为三元组W,R₁,R₂,模型定义为四元组W,R₁,R₂,I;在完备性定理证明过程中,需要在由所有极大协调集所构成的集合上构造出两个等价关系,其典型模型的构建方法与经典典型模型的构建方法不同.如果□₁的可达关系R₁等于□₂的可达关系R₂,那么该逻辑的公理化系统变成S5.     

Impact of bandwidth request schemes for Best-Effort traffic in IEEE 802.16 networks

In IEEE 802.16 networks, bandwidth request–grant schemes are employed for reducing data collision and supporting various QoS requirements. In this paper, we investigate the impact of such schemes on Best-Effort (BE) traffic. We examine three candidate schemes. In the first scheme, each Subscribe Station (SS) attempts to request bandwidth in every frame to reduce delay. In the second scheme, the number of bandwidth request is limited to avoid collisions in bandwidth request. In the third scheme, a base station allocates bandwidth to each SS based on the measured sending rate without explicit bandwidth request. We quantitatively analyze the performance of these schemes in terms of the collision rate and buffer length. We also present a simulation study to validate our analysis and to observe the impact of these schemes on BE traffic. This paper shows that IEEE 802.16 networks can be effectively managed through appropriate bandwidth request schemes. It is also shown that bandwidth allocation without request can be an alternative for increasing utilization.     

The implementation of IEEE 802.16m protocol module for ns-3 simulator

This paper presents the first IEEE 802.16m medium access control (MAC) protocol module embedded in the network simulator version 3 (ns-3). The designed module provides a validation tool for researchers to verify simulated results related to the IEEE 802.16m. This module supports the basic MAC functions which includes an initial ranging (IR) function; four types of bandwidth request (BR) procedures; five types of standard-specified quality-of-service (QoS); a QoS-based transmission scheduler; an adaptive modulation and coding (AMC) selection function; a physical (PHY) layer with a channel propagation model. The designed module provides modification flexibility to adapt to other simulation modules. Various scenarios are simulated to show the capacity of the implemented IEEE 802.16m module in details. Results of different types of traffic are tested with the path loss effect on transmission to visualize the numeric capability of the module.     

An efficient bandwidth allocation algorithm for real-time VBR stream transmission under IEEE 802.16 wireless networks

This paper investigates variable rate control strategies for real-time multimedia variable bit rate (VBR) services over IEEE 802.16 broadband wireless networks. A data rate control mechanism is derived for the case where the uplink channel provides real-time services and the traffic rate parameter remains constant. This paper shows that the common queuing scheduling algorithms have some bandwidth allocation fairness problems for the real-time polling service (rtPS) in the MAC layer. In other words, the use of a VBR for the rtPS by a WiMAX system results in additional access latency jitter and bandwidth allocation disorder in the transmitted multimedia streams during the regular time interval polling of subscribe stations (SSs) for the contention bandwidth request period. However, the proposed scheduling algorithm solves these SSs contending with bandwidth resource allocation problems based on an extended rtPS (ertPS) of quality-of-service (QoS) pre-programming for a ranging response non-contention polling period. The adopted bandwidth allocation of max–min fairness queue scheduling uses a time constraint condition to transmit real-time multimedia VBR streaming in an IEEE 802.16 broadband wireless environment. In addition, we use the ns-2 simulation tool to compare the capacity of multimedia VBR stream and show that the proposed ertPS scheduling algorithm outperforms other rtPS scheduling algorithms.     

Efficiency-driven selection of bandwidth request mechanism in broadband wireless access networks

To avoid collisions in WiMAX networks, the connections in Subscriber Stations (SSs) use a request–grant process to acquire transmission resources from the Base Station (BS). In accordance with the IEEE 802.16 standard, the request–grant process is accomplished using either a unicast polling method or a contention request method. In WiMAX systems, the number of bandwidth-request (BR) slots per frame is limited. Thus, to enhance the network performance, the BR slots must be used in the most efficient manner possible. In practical WiMAX systems, the offered network load varies over time, and thus the strict use of either the unicast polling method or the contention request method results in a poor utilization efficiency of the BR slots. Accordingly, the present study proposes a scheme designated as Efficiency-Driven Selection of Bandwidth Request (EDSBR), in which the request–grant mechanism is adjusted dynamically on a frame-by-frame basis in accordance with the network conditions. The performance of the proposed scheme is evaluated by simulations. The results show that EDSBR achieves a more efficient utilization of the BR slots than the unicast polling scheme or the contention request scheme, and therefore yields an improved network performance.     

Above packet level admission control and bandwidth allocation for IEEE 802.16 wireless MAN

IEEE 802.16 wireless Metropolitan Area Network (MAN) is expected to revolutionize the broadband wireless access technology. Efficient resource management is essential in providing scalability in such large IEEE 802.16-based wireless network and Quality of Service (QoS) for multimedia applications (VoIP, MPEG, FTP, WWW) is usually achieved by appropriate classification of scheduling services and grant/request mechanism. In this paper, we firstly discuss the QoS issue in IEEE 802.16 wireless MANs and propose a dynamic admission control scheme for scheduling services defined in the 802.16 specification. The proposed scheme provides the highest priority for Unsolicited Grant Service (UGS) connections and maximizes the bandwidth utilization by employing bandwidth borrowing and degradation. We develop an approximate analytical model to evaluate the system performance by assuming that the traffic processes of all scheduling services are Poisson processes. In fact, the self-similarity of non-voice traffic makes its traffic process far from Poisson process and should be modeled by Poisson Pareto Burst Process (PPBP). Therefore, in the later part of the paper, we analyze upper bound blocking probabilities of all scheduling services above the packet level using PPBP model for fractal traffic and Gaussian model for aggregated traffic in large wireless network as well as using the Chernoff bound method. Based on the analytical results, we give another admission control and bandwidth allocation mechanism above the packet level so as to minimize the blocking probability of each type of service in IEEE 802.16 wireless MAN. Analytical and simulation results are obtained and compared to demonstrate the effectiveness of proposed schemes and validate our analytical models.     

Simulator for WiMAX networks

The Network Simulator (ns-2) is a popular tool for the simulation of computer networks; it provides substantial support for simulation of Internet protocols over wired and wireless networks. Although some modules for WiMAX networks simulation have been proposed for the ns-2, none of them implements all MAC features specified by the IEEE 802.16 standard for bandwidth management and QoS support. This paper, however, does present the design and validation of a WiMAX module based on the IEEE 802.16 standard. The module implemented includes mechanisms for bandwidth request and allocation, as well as for QoS provision. Moreover, the implementation is standard-compliant.     

基于模数的WiMAX网络高负载自适应冲突解决算法

根据现有的部分研究结果发现,IEEE 802.16标准中使用的截断二进制指数后退算法在解决网络冲突时效率很低;并且当网络的负载增多时,该算法的运行性能更差.在此基础上,提出了一种基于模数的自适应冲突解决算法来优化WiMAX网络的冲突解决过程.该算法将所有用户站按照当前时间帧内的传输机会数量分为多个小组,并且要求用户站只能通过特定的传输机会发送请求.与此同时,为了保证整个网络的高性能,本算法要求基站和用户站根据上一轮请求过程中的平均冲突概率和传输机会使用率来调整当前时间帧内的传输机会数量,以及挑选部分用户站参与带宽请求,保证了WiMAX网络在高负载情况下的运行性能.模拟结果表明,本算法在WiMAX网络运行过程中能够较好地解决请求过程中的冲突,对网络性能提高较大.     

A performance study of uplink scheduling algorithms in point-to-multipoint WiMAX networks

The IEEE 802.16 standard defines the specifications for medium access control (MAC) and physical (PHY) layers of WiMAX networks. A critical part of the MAC layer specification is packet scheduling, which resolves contention for bandwidth and determines the transmission order of users. Evaluating the performance packet scheduling algorithms is of utmost importance towards realizing large-scale WiMAX deployment. In this paper, we conduct a comprehensive performance study of scheduling algorithms in point-to-multipoint mode of OFDM-based WiMAX networks. We first make a classification of WiMAX scheduling algorithms, then simulate a representative number of algorithms in each class taking into account that vital characteristics of the IEEE 802.16 MAC layer and OFDM physical layer. We evaluate the algorithms with respect to their abilities to support multiple classes of service, providing quality of service (QoS) guarantees, fairness amongst service classes and bandwidth utilization. To the best of our knowledge, no such comprehensive performance study has been reported in the literature. Simulation results indicate that none of the current algorithms is capable of effectively supporting all WiMAX classes of service. We demonstrate that an efficient, fair and robust scheduler for WiMAX is still an open research area. We conclude our study by making recommendations that can be used by WiMax protocol designers.     

Uplink Scheduling for Supporting Real Time Voice Traffc in IEEE 802.16 Backhaul Networks

In this paper we propose simple enhancements to the bandwidth (BW) request messages in IEEE 802.16 for supporting real-time packet voice traffc. Three different BW request formats are proposed, each requiring a different amount of latency information about the buffered packets at the SS. On this basis, packet scheduling schemes are proposed for the BS to make resource allocations for real-time traffc. Our results show that the proposed BW request and scheduling schemes achieve significantly lower packet loss probability than the standard IEEE 802.16 BW request with round robin scheduling. The results further show that there is an optimum point about how much delay information the SS should report to the BS in order to best utilize the uplink resources while the SS provides satisfactory real-time performance for the voice traffc.     

Scheduling solution for the IEEE 802.16 base station

The IEEE 802.16 standard defines a wireless broadband access network technology called WiMAX. It introduces several advantages, one of which is the support for QoS at the MAC level. To ensure meeting the QoS requirements, the 802.16 base station must run some algorithm to allocate slots between connections. This algorithm is not defined in the 802.16 specification but rather is open for alternative implementations. We propose a simple, yet efficient, solution that is capable of allocating slots based on the QoS requirements, bandwidth request sizes, and the 802.16 network parameters. To test the proposed solution, we have implemented the 802.16 MAC and PHY layers in the NS-2 simulator. Several simulation scenarios are presented that demonstrate how the scheduling solution allocates resources in various cases. According to the simulation results, the proposed scheduling solution ensures the QoS requirements of all 802.16 service classes. The solution shares free resources fairly and demonstrates work-conserving behaviour.